Reversible Capture and Release of a Ligand Mediated by a Long-Range Relayed Polarity Switch in a Urea Oligomer

Ethylene-bridged oligoureas characterized by a continuous, switchable chain of hydrogen bonds and carrying a binding site (an N,N′-disubstituted urea) for a hydrogen-bond-accepting ligand (a phosphine oxide) were synthesized. These oligomers show stronger ligand binding when the binding site is located at the hydrogen-bond-donating terminus than when the same binding site is at the hydrogen-bond-accepting terminus. An acidic group at the terminus remote from the binding site allows hydrogen bond polarity, and hence ligand binding ability, to be controlled remotely by a deprotonation/reprotonation cycle. Addition of base induces a remote conformational change that is relayed through up to five urea linkages, reducing the ability of the binding site to retain an intermolecular association to its ligand, which is consequently released into solution. Reprotonation returns the polarity of the oligomer to its original directionality, restoring the function of the remote binding site, which consequently recaptures the ligand. This is the first example of a synthetic molecular structure that relays intermolecular binding information, and these “dynamic foldamer” structures are prototypes of components for chemical systems capable of controlling chemical function from a distance.


General Information
Where specified, procedures were performed under an atmosphere of nitrogen. Air and moisturesensitive liquids/solutions were transferred to reaction vessels by syringe under an atmosphere of nitrogen. Solvents and reagents were purchased from commercial suppliers and were used without further purification unless otherwise specified. Agitation was achieved using Teflon coated stirrer bars by magnetic induction. All thin layer chromatography (TLC) experiments were conducted on precoated plastic plates (Macherey-Nagel polygram SIL G/UV 254 ) and visualized using ultraviolet light (254 nm) or staining. Flash chromatography was performed on an automated Biotage Isolera TM Spektra Four using gradient elution on pre-packed silica gel Sfär Duo columns. Solvent systems for TLC and flash chromatography are reported in solvent:solvent volume ratios. All variable-temperature NMR experiments were conducted using a Bruker AVANCE III HD 500 MHz NMR Spectrometer with 5 mm DCH 13 C-1 H/D Cryo Probe (500 MHz). All room temperature NMR experiments were conducted using a Bruker Nano 400 Spectrometer (400 MHz) or a Bruker AVANCE III HD 500 MHz NMR Spectrometer with 5 mm DCH 13 C-1 H/D Cryo Probe (500 MHz), with chemical shifts reported (δ in ppm) relative to the specified deuterated solvent. All 31 P NMR spectra are referenced relative to an external standard

N-(3,5-Bis(trifluoromethyl)phenyl)-N'-butyl urea, 3
To a solution of BuNH 2 (36.6 mg, 0.50 mmol, 1.0 was added and the product was extracted with CH 2 Cl 2 (30 mL + 20 mL) then the combined organic extracts were dried (Na 2 SO 4 ) and concentrated. To remove the di-urea side product, the residue was dissolved in Et 2 O (10 mL) and 1 M aqueous HCl (5 mL) and water (10 mL) were added. The biphase was stirred at room temperature for 20 min, then diluted with Et 2 O (25 mL) and 1 M aqueous HCl (3 mL) and water (20 mL). The organic phase was separated. The aqueous phase was basified (to pH > 12) with NaOH pellets, then the product was extracted with CH 2 Cl 2 (30 mL + 20 mL). The combined organic extracts were dried (Na 2 SO 4 ) and concentrated. Flash chromatography (Biotage, 10 g Sfär Duo column, MeOH/CH 2 Cl 2 gradient from 0:100 to 6. 5

Conformational Analysis
Figure S1 -Variable temperature 1 H NMR spectra of compound 1 (500 MHz, 27 mM, CD 2 Cl 2 ). Two conformers are present, which differ in the directionality of the hydrogen bond chain. The major conformer 1 (75% at −10 ºC) has the thiourea at the hydrogen bond-accepting terminus, while the minor conformer 1' (25% at −10 ºC) has the thiourea at the hydrogen bond-donating terminus. Selected pairs of rotationally exchanging protons are highlighted in different colours (except for the internal urea N-Hs, which are all coloured yellow), and where the protons in rotational exchange resonate at distinct chemical shifts, the relevant signals are labelled on the spectra as belonging to the major or minor conformer. These assignments were supported by NOESY (EXSY) and ROESY experiments (vide infra). The disappearance of the benzylic methylene signal at ~5.3 ppm (coloured purple) for the major conformer below −20 °C is attributed to the proximal sulfur atom slowing down rotation about the N−CH 2 Ph bond at lower temperatures, causing signal broadening.               Two conformers are present, which differ only in the local conformation of the disubstituted urea − the global directionality of the hydrogen bond chain is the same in both conformers, with the disubstituted urea occupying the hydrogen bond-accepting terminus. The major conformer 2 (95% at −10 ºC) has the disubstituted urea in the syn,syn-conformation and its alkyl N-H participates in a seven-membered hydrogen-bonding ring with the adjacent urea. In the minor conformer 2' (5% at −10 ºC), the disubstituted urea is in an anti,syn-conformation and only the aryl N-H participates in intramolecular hydrogen bonding; in this case in a nine-membered ring. Selected pairs of rotationally exchanging protons are highlighted in different colours (except for the internal urea N-Hs, which are all coloured yellow), and where the protons in rotational exchange resonate at distinct chemical shifts, the relevant signals are labelled on the spectra as belonging to the major or minor conformer. These assignments were supported by a NOESY (EXSY) experiments (vide infra).    Like its shorter homologue 1, two conformers are populated for 7, which differ in the directionality of the hydrogen bond chain. The major conformer 7 (74% at −10 ºC) has the thiourea at the hydrogen bond-accepting terminus, while the minor conformer 7' (26% at −10 ºC) has the thiourea at the hydrogen bond-donating terminus. Selected pairs of rotationally exchanging protons are highlighted in different colours (except for the internal urea N-Hs, which are all coloured yellow), and where the protons in rotational exchange resonate at distinct chemical shifts, the relevant signals are labelled on the spectra as belonging to the major or minor conformer. These assignments were supported by NOESY (EXSY) experiments (vide infra). The disappearance of the benzylic methylene signal at ~5.2 ppm (coloured purple) for the major conformer below −10 °C is attributed to the proximal sulfur atom slowing down rotation about the N−CH 2 Ph bond at lower temperatures, causing signal broadening.   Cross-peaks arising from through space correlations (nOe) appear in the same phase as the cross peaks from rotational exchange (EXSY correlations), indicating that the nOes are negative.
Figure S16 -Variable temperature 1 H NMR spectra of compound S10 (500 MHz, 25 mM, CD 2 Cl 2 ). Like 1, two conformers are populated for alkyl urea analogue S10, which differ in the directionality of the hydrogen bond chain. The major conformer S10 (72% at −10 ºC) has the thiourea at the hydrogen bond-accepting terminus, while the minor conformer S10' (28% at −10 ºC) has the thiourea at the hydrogen bond-donating terminus. Selected pairs of rotationally exchanging protons are highlighted in different colours (except for the internal urea N-Hs, which are all coloured yellow), and where the protons in rotational exchange resonate at distinct chemical shifts, the relevant signals are labelled on the spectra as belonging to the major or minor conformer. These assignments were supported by NOESY (EXSY) experiments. The disappearance of the benzylic methylene signal at ~5.2 ppm (coloured purple) for the major conformer below −20 °C is attributed to the proximal sulfur atom slowing down rotation about the N−CH 2 Ph bond at lower temperatures, causing signal broadening.
Figure S16a -Selective one-dimensional NOESY experiment for compound S10 at −10 ºC (500 MHz, 25 mM, CD 2 Cl 2 ). (Upper) 1 H NMR spectrum at −10 ºC (for reference); (Lower) Irradiation of the thiourea proton of the major conformer (δ H = 11.14 ppm, coloured blue). Signals arising from rotational exchange and through space nOes appear in the same phase, indicating that the nOes are negative. 'Exchange nOes' are also observed as a result of excitation transfer between the two conformers due to rotational exchange occurring on the timescale of the nOe build-up.
Table S1 -Summary of the conformational populations of thioureas 1, 7 and S10 under various conditions. All ratios were measured at −10 °C. The capture and release experiments shown in the manuscript essentially involve the 'major' conformer of 1 and 7 (i.e., conformer X, drawn on the left of the equilibrium below). As such, we investigated the effect of various parameters on the conformational population of X. The results show that the length of the oligomer (entry 1 versus entry 2), the identity of the internal ureas (entry 1 versus entry 3) and the concentration (entries 1, 4 and 8) have little effect on the conformer ratio, favouring X in all cases. Adding the ligand (Bu 3 PO) at the same concentration as the capture and release experiments further shifts the equilibrium in favour of X (entry 4 versus entry 5), consistent with the proposed binding at the BTMP urea. Finally, the conformer ratio was found to be dependent on the solvent (entries 6−11): an almost equal population of the two conformers was observed in CDCl 3 (entry 7), while X was strongly favoured by more polar solvents and/or solvents with hydrogen bond-accepting heteroatoms (entries 9−11).           a Bu 3 PO (2 mM) was also present. b Broad, unresolved signals were observed. c Partial precipitation was observed upon cooling in the NMR spectrometer.                 In this alternative orientation, an NOE would likely be expected between the urea NH (colored yellow) and the BTMP thiourea ortho-aryl protons (colored grey), as well as between the ortho protons on each of the two N-aryl rings; however, no appreciable NOE was observed in either case. These results tentatively suggest that it is the sulfur that points inwards and hydrogen bonds to the adjacent urea proton − a situation that would also relieve steric interactions between the N-aryl rings. • 1 H NMR spectra were also acquired for every experiment to confirm correct stoichiometry and, where applicable, chemoselective thiourea deprotonation; hence, why CD 2 Cl 2 was used as the 31 P NMR solvent instead of CH 2 Cl 2 ( 1 H NMR spectra were also acquired with the Ph 3 PO external standard capillary in the NMR tube).

Titrations, Binding Constants and Related Experiments
• The major indicators of deprotonation of the thiourea function in 1, 4, 6 and 7 with t-BuN=P(NMe 2 ) 3 were the loss of the thiourea NH signal at ca 11 ppm in the 1 H NMR spectrum, as well as a new signal in the 31 P NMR spectrum (in addition to Bu 3 PO) corresponding to [t-BuHN−P(NMe 2 ) 3 ] + •X − at δ P = 32−35 ppm (X = 1, 4, 6 or 7). For comparison, the conjugate base t-BuN=P(NMe 2 ) 3 was found to have δ P = 7.05 ppm under the same conditions ( Figure S28).  Figure  S29).
• Although no attempts have been made to isolate the [t-BuHN−P(NMe 2 ) 3 ] + salts of thiourea anions 1 − , 4 − , 6 − or 7 − , we have seen no evidence of sensitivity to air or adventitious moisture during the timeframes of NMR analysis of their CD 2 Cl 2 solutions. We note, however, that they do undergo slow alkylation at sulfur by the solvent itself (CD 2 Cl 2 ). For this reason, all 1 H and 31 P NMR spectra of these anions (1 − , 4 − , 6 − or 7 − ) were acquired within 30 min of adding the base (t-BuN=P(NMe 2 ) 3 ). Subsequent reprotonation, where relevant, was then carried out immediately after recording the NMR spectra. With these precautions taken (as described in the general procedure), at most, traces of thiourea alkylation are observed. The rate of alkylation was nonetheless investigated by 1  The resulting solution was transferred quantitatively to an NMR tube and a sealed capillary tube containing Ph 3 PO (150 mM in CD 2 Cl 2 ) was placed inside. 1 H and 31 P{ 1 H} NMR spectra were acquired.
• To ensure the volume in the NMR tube was maintained at 0.50 mL throughout these experiments (and hence a constant 2.0 mM concentration of Bu 3 PO), a 'reference NMR sample' was prepared containing 0.50 mL of CH 2 Cl 2 and an empty capillary tube (the same size as used for the external standard); a line was marked on this tube at the solvent level as a reference.